For most adults, a temperature at or above 100.4°F (38°C) is considered a fever. But “high” is relative, and the number that matters depends on how you’re measuring, the time of day, and your age. A reading over 104°F (40°C) is considered a high fever that warrants a call to your doctor.
What Counts as a Fever
There’s no single universal cutoff. Different medical references define fever slightly differently, which can be confusing when you’re staring at a thermometer. The Merck Manual defines fever as an oral temperature above 100°F (37.8°C) or a rectal temperature above 100.8°F (38.2°C). Harrison’s Principles of Internal Medicine uses a broader range: a core (rectal) temperature between 99.5°F and 100.9°F (37.5°C to 38.3°C), with oral thresholds that shift depending on the time of day. In the morning, an oral reading above 99°F (37.2°C) qualifies. By late afternoon, the threshold rises to about 99.9°F (37.7°C).
The most commonly cited number across emergency departments and clinical guidelines is 100.4°F (38°C). If your thermometer reads at or above that, you have a fever by virtually any definition.
Fever Severity by Temperature Range
Not all fevers carry the same level of concern. One of the earliest clinical classification systems, still broadly reflected in modern practice, breaks fever into tiers:
- Low-grade fever: 100.4°F to 101.1°F (38°C to 38.4°C). This is the most common range during mild infections like a cold or mild flu. Your body is mounting a normal immune response.
- Moderate fever: 101.3°F to 102.2°F (38.5°C to 39°C). You’ll likely feel noticeably unwell, with chills, body aches, or fatigue.
- High fever: 103.1°F to 104.9°F (39.5°C to 40.5°C). This range warrants close attention and usually a call to your doctor, especially if it persists.
- Dangerous fever (hyperpyrexia): Above 106.7°F (41.5°C). This is a medical emergency. According to Cleveland Clinic, temperatures this high can cause permanent organ damage and can be life-threatening.
Harvard Health Publishing recommends calling your doctor for any fever over 104°F (40°C). You don’t need to wait for the hyperpyrexia threshold to take a fever seriously.
Why 98.6°F Isn’t Really “Normal”
The 98.6°F benchmark dates back to the 1800s, and while it’s still widely cited, it’s more of an average than a rule. Normal body temperature actually ranges from about 97°F to 99°F (36.1°C to 37.2°C) depending on the person. Your individual baseline might sit comfortably at 97.4°F or 98.8°F, and both are perfectly healthy.
Body temperature also follows a daily rhythm. It’s lowest around 4 a.m. and peaks around 6 p.m., with swings of up to 0.9°F (0.5°C) in either direction. This means a reading of 99.5°F at 7 a.m. is more significant than the same number at 5 p.m. It also explains why fevers tend to feel worse at night: your baseline is already climbing toward its daily peak, and the fever pushes it higher.
Where You Measure Matters
Different thermometer placements give different readings. Rectal temperatures run about 0.8°F (0.43°C) higher than underarm (axillary) readings. Oral temperatures fall in between, roughly 0.5°F (0.25°C) above an armpit reading. So a 100°F reading under the arm doesn’t mean the same thing as 100°F taken orally.
If you’re using an underarm thermometer, a reading of 99.5°F (37.5°C) corresponds roughly to an oral fever of 100.4°F. A reading of 99°F (37.2°C) under the arm is roughly equivalent to a rectal temperature of 100.4°F. Most fever guidelines assume oral measurement, so if you’re using a different method, you’ll need to mentally adjust.
Fever Thresholds Are Lower for Older Adults
Adults over 65 run cooler baselines and mount weaker fever responses. This means a temperature that looks unremarkable on paper can signal a serious infection in an older person. Research in Clinical Infectious Diseases found that using the standard 100.4°F oral cutoff caught only 40% of infections in nursing home residents. Dropping the threshold to 99°F (37.2°C) detected 83% of infections.
For older adults, a persistent oral temperature at or above 99°F, or any increase of 1.3°F or more above their personal baseline, should be treated as a potential fever regardless of the absolute number. This is especially important because infections like pneumonia and urinary tract infections are both more common and more dangerous in this age group, yet they can progress with only a modest temperature rise.
When to Treat a Fever
A fever is your immune system working, and not every fever needs medication. The Mayo Clinic’s guidance for adults is straightforward: for oral temperatures up to 102°F (38.9°C), rest and fluids are enough. You don’t need fever-reducing medication unless you’re uncomfortable. Above 102°F, over-the-counter options like acetaminophen, ibuprofen, or aspirin can help bring the temperature down and ease symptoms like headache and body aches.
There are situations where a fever at any level deserves prompt medical attention: if it lasts more than three days, if it comes with a stiff neck, severe headache, confusion, difficulty breathing, or a rash, or if you have a weakened immune system from conditions like cancer treatment or organ transplant. A fever above 104°F (40°C) on its own is reason enough to call your doctor, even without other symptoms.